Practical takeaway
Resonant circuits exchange energy between electric and magnetic fields. Q describes how strongly energy is stored relative to how quickly it is lost.
E5A · Amateur Extra question group
Resonance, Q & Bandwidth
Resonant circuits exchange energy between electric and magnetic fields. Q describes how strongly energy is stored relative to how quickly it is lost.
Amateur Extra exam focus
The explanation below teaches the ideas behind E5A while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. Resonance occurs when reactances cancel
At resonance, inductive and capacitive reactance are equal in magnitude and opposite in sign. A series resonant circuit can present low impedance; a parallel resonant circuit can present high impedance.
2. Q links loss and selectivity
Higher Q means stored energy is large compared with energy lost per cycle. In many tuned circuits, higher Q produces narrower bandwidth and larger circulating voltages or currents.
3. Loaded Q includes the external circuit
A resonator by itself has an unloaded Q. Coupling to a source and load adds loss and lowers effective loaded Q, so filter bandwidth depends on both component quality and the way the circuit is connected.
E5A checklist
- At resonance, inductive and capacitive reactance cancel.
- Higher Q generally means lower loss and narrower bandwidth.
- High-Q circuits can develop large circulating voltage/current.
- Loaded Q includes source/load coupling losses.
Exam scope: NCVEC 2024-2028 Amateur Extra Class (Element 4), effective July 1, 2024 through June 30, 2028, with February 4, 2026 fourth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool